Scan testing system, method and apparatus

ABSTRACT

Test circuits located on semiconductor die enable a tester to test a plurality of die/ICs in parallel by inputting both stimulus and response patterns to the plurality of die/ICs. The response patterns from the tester are input to the test circuits along with the output response of the die/IC to be compared. Also disclosed is the use of a response signal encoding scheme whereby the tester transmits response test commands to the test circuits, using a single signal per test circuit, to perform: (1) a compare die/IC output against an expected logic high, (2) a compare die/IC output against an expected logic low, and (3) a mask compare operation. The use of the signal encoding scheme allows functional testing of die and ICs since all response test commands (i.e. 1-3 above) required at each die/IC output can be transmitted to each die/IC output using only a single tester signal connection per die/IC output. In addition to functional testing, scan testing of die and ICs is also possible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/626,201,filed Jan. 23, 2007, now pending;

which was a divisional of application Ser. No. 11/103,781, filed Apr.11, 2005, now U.S. Pat. No. 7,183,570, issued Feb. 27, 2007;

which was a divisional of application Ser. No. 10/301,898, filed Nov.22, 2002, now U.S. Pat. No. 6,894,308, issued May 17, 2005;

which claims priority under 35 USC 119(e)(1) of provisional applicationSer. No. 60/333,803, filed Nov. 28, 2001.

FIELD OF THE DISCLOSURE

As the geometry of semiconductor transistors continue to shrink, moreand more functional circuitry may be embedded within integrated circuits(ICs). This trend is beneficial for the electronics industry since itenables development of smaller, lower power electronic consumerproducts, such as cell phones and hand held computers. However, as ICcircuit density increases, the testing of ICs becomes more complex andcostly for the IC manufacturers. Reducing the cost of manufacturing ICsis a primary goal for every IC manufacturer. By reducing ICmanufacturing cost, an IC manufacturer can advantageouslycost-differentiate its IC products from other IC manufacturers.Therefore, an IC manufacturer that continually strives to create newmethods and processes for improving and reducing the cost of IC testingwill, in the long run, be successful over IC manufacturers that maintainuse of conventional IC testing methods and processes. The presentdisclosure describes a novel method and apparatus that improves uponconventional IC test methods and processes in use today.

DESCRIPTION OF THE RELATED ART

FIG. 1A illustrates a semiconductor wafer 101 comprising multiple die102 circuits. FIG. 1B illustrates one of the die circuits 101 on wafer101. The die contains core circuitry 103 that provides the functionalityof the die, and pad locations 104 for providing contacts for accessingthe core circuitry.

FIG. 1 c illustrates a test arrangement for contacting and testing asingle die 102 of wafer 101. The test arrangement includes a tester 105,a single die probe mechanism 109, and a die 102 to be tested. Tester 105comprises a controller 105, stimulus circuitry 108, and responsecircuitry 107. Controller 106 regulates the stimulus circuitry 108 viainterface 117 to output test stimulus signals to die 102 via stimulusbus 111. Controller 106 regulates the response circuitry 107 viainterface 118 to receive test response signals from die 102 via responsebus 110.

Probe mechanism 109 comprises the stimulus bus 111 and response bus 110connection channels between tester 105 and die 102. The probe mechanismcontacts the input 115 and output 116 die pads via small probe needles112. While only a pair of input and output probe needles 112 are shownin this simple illustration, it is understood that all die input andoutput pads will be similarly contacted by the probe mechanism 109 usingadditional probe needles 112. The input pads 115 transfer stimulussignals to core 103 via input buffers 113, and the output pads 116transfer test response signals from core 103 via output buffers 114. Thetesting of the die 102 in FIG. 1C occurs through the process ofinputting stimulus signals to the die and receiving response signalsfrom the die.

FIG. 2 illustrates in more detail the stimulus 108 and response 107circuitry of tester 105. Stimulus circuitry 108 typically comprises alarge stimulus data memory 201 for storing the stimulus data to beapplied to the die. Controller 106 controls the loading of the stimulusdata memory 201 from a source, such as a hard disk, prior to testing,and then controls the stimulus data memory to output the loaded stimulusdata to the die during test, via stimulus bus 111. Response circuitry107 typically comprises a large mask and expected data memory 203, acomparator 204, and a fail flag memory 202. The mask and expected datamemory 203 stores mask and expected data to be used by the comparator204 to determine if the response data from the die passes or fails.

During test, the comparator 204 inputs response signals from the die viaresponse bus 110, and mask (M) and expected (E) data signals from memory203 via mask and expected data buses 206 and 207. If not masked, by masksignal input from memory 203, a given response signal from the die iscompared against a corresponding expected data signal from memory 203.If masked, by mask signal input from memory 203, a given response signalfrom the die is not compared against an expected data signal from memory203. If a non-masked response signal matches the expected signal, thecompare test passes for that signal. However, if a non-masked responsesignal does not match the expected signal, the compare test fails forthat signal and the comparator outputs a corresponding fail signal onbus 205 to fail flag memory 202. At the end of test, the controller 106reads the fail flag memory to determine if the die test passed orfailed.

Alternately, and preferably in a production test mode, the single dietest may be halted immediately upon the controller receiving a comparefail indication from the fail flag memory 202, via the interface 118between controller 106 and response circuitry 107, to reduce wafer testtime. At the end of the single die test, the probe mechanism isrelocated to make contact to another single die 102 of wafer 101 and thesingle die test is repeated. The wafer test completes after all die 102of wafer 101 have each been contacted and tested as described above.

FIG. 3 illustrates a test arrangement for simultaneously contacting andtesting multiple die 102 of wafer 101. The test arrangement includestester 105, multiple die probe mechanism 301, and a multiple die 1-N 102to be tested. The difference between the single and multiple die testarrangements of FIGS. 2 and 3 is in the use of the multiple die probemechanism 301. As seen in FIG. 3, the connection between probe mechanism301 and tester 105 is as previously described. However, the connectionbetween probe mechanism 301 and die 1-N is different. Each stimulus bussignal from the tester uniquely probes common pad inputs on each die1-N. For example, the stimulus 1 (S1) signal from the stimulus busprobes all common input pads 303 of all die 1-N via connection 302.While not shown, stimulus 2-N (S2-N) signals from the stimulus bus wouldeach similarly probe all other common input pads of all die 1-N. Thisallows the stimulus bus signals to simultaneously input the samestimulus to all die 1-N during the test.

As seen in FIG. 3, the die response connection of probe mechanism 301 isdifferent from the above described die stimulus connection. Whereas eachcommon input pad 303 of die 1-N share a single stimulus signalconnection 302, each common output pad 304 requires use of a dedicatedresponse signal connection. For example, output pad 304 of die 1 uses aresponse signal connection 305, output pad 304 of die 2 uses a responsesignal connection 306, output pad 304 of die 3 uses a response signalconnection 307, and output pad 304 of die N uses a response signalconnection 108. All other output pads of die 1-N would similarly use adedicated response signal connection. All dedicated response signalconnections are channeled into the response bus to tester 105, as seenin FIG. 3.

During test, the tester outputs stimulus to all die 1-N and receivesresponse outputs from all die 1-N. The test time of testing multiple diein FIG. 3 is the same as testing single die in FIG. 2. The test operatesin the masked/non-masked compare mode as described in FIGS. 1C and 2.When testing multiple die simultaneously, as opposed to testing a singledie, a production test preferably runs to completion even though anearly compare may occur on one or more of the die being tested. This isdone because typically most of the die will pass the production test andaborting the multiple die production tests on a failure indication wouldactually increase the test time, since the test would need to be re-runlater to complete the testing of the passing die.

The limitation of the multiple die test arrangement in FIG. 3 lays inthe number of dedicated response inputs 305-308 the tester 105 canaccept on its response bus. For example, if the tester can accept 300response input signals and each die has 100 output pads, the multipledie test arrangement of FIG. 3 is limited to only being able to test 3die at a time. Testing 300 die on a wafer with this 3 die per testlimitation would required having to relocate the probe mechanism 301approximately 100 times to contact and test three die at a time. Thetime required to relocate the probe mechanism and repeat the die testsay 100 times consumes test time, which increases the cost tomanufacturer the die. It is possible to widen the response bus input ofthe tester to say 600 inputs to allow testing 6 die at a time, butadding circuitry to the tester to increase its response bus input widthis expensive and that expense would increase the cost of manufacturingdie.

The present disclosure, as described in detail below, providesimprovements that overcome the limitations stated above usingconventional multiple die testing arrangements. Most notably, thepresent disclosure provides for significantly increasing the number ofdie that may be tested in parallel, without having to increase the widthof the tester's response bus.

SUMMARY OF THE DISCLOSURE

The present disclosure improves multiple die testing by; (1) adaptingtesters to communicate with multiple die using a novel responsesignaling technique, and (2) adapting the die to be receptive to thetester's novel response signaling technique. Also, the presentdisclosure improves connectivity to multiple die on wafer by processingstimulus and response interconnects on the wafer to improve access tomultiple die during test. In addition to its ability to improve thetesting of multiple die on wafer, the present disclosure may also beused advantageously to improve the testing of multiple packaged ICs.

The present disclosure uses test circuits located on the die to enable atester to test a plurality of die/ICs in parallel by inputting bothstimulus and response patterns to the plurality of die/ICs. The responsepatterns from the tester are input to the test circuits along with theoutput response of the die/IC to be compared. The response patternsinclude one of expected data and mask data input on an output pad of thedie/IC and the other of expected data and mask data input on another padof the die/IC, which may be an input pad or an output pad. In additionto functional testing, scan testing of die and ICs is also possible.

If only scan testing, not functional testing, is desired on die and ICs,the present disclosure may use binary input signaling (i.e. two logicalstates per signal) of response data (mask and expected data) from thetester to the test circuits rather than trinary input signaling (i.e.three logical states per signal) of response data from the tester to thetest circuits. The reason this is possible during scan testing ofdie/ICs, is that typically only a subset of the die/ICs overallfunctional inputs and/or outputs need to be coupled to the tester duringthe test.

Thus during scan testing an ample number of additional die/IC inputsand/or outputs may be available for inputting the previously describedmask and expected data signals to the die/IC using separate signal pathsfor each, instead of encoding each into a single signal path. Thefollowing description details the use of separate mask and expected datasignaling paths to achieve the testing of plural die/ICs.

BRIEF DESCRIPTION OF THE VIEW OF THE DRAWING FIGURES

FIGS. 1A, 1B and 1C are block diagrams, respectively, of a semiconductorwafer containing plural dies, an individual semiconductor die, and asemiconductor tester connected to a die;

FIG. 2 is a block diagram of the parts of the tester;

FIG. 3 is a block diagram of a tester connected to plural dies;

FIG. 4 is a block diagram of a semiconductor tester according to thepresent disclosure;

FIGS. 5A and 5B are, respectively, a block diagram of mask and expectedencoding circuit in the tester of FIG. 4 and a truth table of thatcircuit;

FIGS. 6A and 6B are, respectively, a block diagram of a semiconductordie according to the present disclosure, and a block diagram of a testcircuit substituted for the conventional two state output buffer circuiton that die;

FIGS. 7A, 7B, and 7C are, respectively, a block diagram of the comparecircuit of FIG. 6A, a truth table for that circuit and a block diagramof a pass/fail scan memory of that circuit;

FIGS. 8A and 8B are, respectively, a block diagram of a trinary circuitused in the circuit of FIG. 7A, and a truth table for that circuit;

FIGS. 9A and 9B are, respectively, a block diagram of a semiconductordie according to another aspect of the present disclosure and a blockdiagram of test circuits substituted for the conventional three stateoutput buffer circuit on that die;

FIGS. 10A and 10B are, respectively, a block diagram of the comparecircuit of FIG. 9B and a truth table of that circuit;

FIGS. 11A and 11B are, respectively, a block diagram of a semiconductordie according to another aspect of the present disclosure and a blockdiagram of test circuits substituted for the conventional input/outputbuffers on that die;

FIGS. 12A and 12B are, respectively, a block diagram of the comparecircuit of FIG. 11B and a truth table of that circuit;

FIGS. 13A and 13B are, respectively, a block diagram of a pass/fail scanmemory according to another aspect of the present disclosure and a blockdiagram of a test system including a bussed fail output lead;

FIG. 14 is a block diagram of a test system according to another aspectof the present disclosure;

FIG. 15 is a block diagram of an alternate view of the test system ofFIG. 14;

FIG. 16 is a block diagram of the functional testing of the test systemof FIG. 15;

FIG. 17 is a block diagram of parallel scan testing in the test systemof FIG. 15;

FIGS. 18A and 18B are, respectively, a block diagram of a semiconductordie including multiple embedded IP core sub-circuits according toanother aspect of the present disclosure and a block diagram of a testcircuit substituted for an output buffer circuit of that die;

FIG. 19 is a block diagram of a test system including a semiconductorwafer including multiple dies and a tester;

FIG. 20 is a block diagram of another test system for multiple packagedintegrated circuits, according to the process described in FIG. 14;

FIG. 21 is a block diagram of a tester similar to that of FIG. 4 with adifferent mask and expected driver circuitry;

FIGS. 22A and 22B are, respectively, a block diagram of oneimplementation of a mask driver and an expected data driver, and a truthtable;

FIG. 23 is a block diagram of an output buffer and a test circuitconnected between cores and output pads;

FIG. 24 is a block diagram of an input buffer connected to an input padand a test circuit connected to an output pad;

FIGS. 25A and 25B are, respectively, block diagrams of alternate maskand expected data pad connections to test circuits;

FIGS. 26A, 26B, and 26C are, respectively, a block diagram of oneexample of a compare circuit, a truth table and a block diagram of apass/fail scan memory;

FIG. 27 is a block diagram of the test system of FIG. 15 modified foruse with the modified tester of FIG. 21 and the modified test circuitsof FIGS. 23, 24, 25A and 25B;

FIG. 28 is a block diagram of the parallel scan testing of dies of FIG.27;

FIG. 29 is a block diagram of parallel scan testing of integratedcircuits according to the arrangement of FIG. 27; and

FIG. 30 is a block diagram of the test circuit of FIG. 23 modified fortesting multiple embedded IP cores.

DETAILED DESCRIPTION

Adapting Testers

FIG. 4 illustrates a tester 401 adapted according to the presentdisclosure. Tester 401 is similar to tester 105 in that it includes acontroller 402 similar to controller 106, stimulus circuitry 403 similarto stimulus circuitry 108, and response circuitry 404. Controller 402 isconnected to stimulus 403 and response 404 circuitry via interfaces 414and 415 respectively. Response circuitry 404 includes the previouslydescribed response circuitry section 107 and a new response circuitrysection 405. Response circuitry 405 is the previously mentionedadaptation of the tester to support the new response signaling techniquefor testing multiple die according the present disclosure.

Response circuitry 405 comprises an enable, mask, and expected datamemory 406, and mask (M) and expected (E) data encoding circuitry 407.Memory 406 outputs a mask (MSK) data bus 410, expected (EXP) data bus409, and an enable (ENA) bus 408 to encoding circuitry 407. Encodingcircuitry 407 outputs an encoded response bus 411. The encoded responsebus 411 of response circuitry 405 differs from the response bus 110 ofresponse circuitry 107 in that the encoded response bus 411 is an outputbus and the response bus 110 is an input bus. Both response busses 411and 110 reside on the input/output bus 413 to response circuitry 404.The role of response bus 110 of circuit 107 is reduced when using tester401 to test multiple die according to the present disclosure, as will bedescribed later. Memory 406 of response circuitry 405 is accessed by thecontroller 402 via interface 415 to load data into memory 406 prior totesting, and to operate the memory 406 to output mask, expected, andenable data to encoding circuitry 407 during test.

FIG. 5A illustrates an example of one of a plurality of mask andexpected data encoding circuits 501 existing within the mask andexpected data circuit 407. Circuit 501 receives a mask data signal 512from bus 410, an expected data signal 513 from bus 409, and an enablesignal 514 from bus 408, and outputs an encoded response signal 511 onbus 411. The mask 512 and expected 513 data signals are input to decoder501. Decoder 501 decodes the mask and expected signal inputs and outputscontrol signals 506-508 to the control input terminal of switches, forexample transistors, 503-505. One contact terminal of switch 503 isconnected to a ground reference voltage (Gnd) and the other terminalcontact is connected to the input 509 of voltage follower amplifier 510.One contact terminal of switch 504 is connected to a positive referencevoltage (Vdd) and the other terminal contact is connected to the input509 of voltage follower amplifier 510. One contact terminal of switch505 is connected to a mid-point reference voltage between Vdd and Gnd(½Vdd) and the other terminal contact is connected to the input 509 ofvoltage follower amplifier 510. Amplifier 510 receives the enable input514 to enable or disable its output.

In FIG. 5B, the operation of encoding circuit 501 is best understood viaa truth table. When the enable input (ENA) 514 is low, the output ofamplifier 510 is disabled from driving the encoded response output 511.When ENA 514 is high, the encoded response 511 output modes of circuit501 are; (1) Gnd (Low) when mask data input 512 (MSK)=0 and expecteddata input 513 (EXP)=0, (2) Vdd (High) when MSK=0 and EXP=1, and (3)½Vdd (Mask) when MSK=1. So, the encoding circuit 501 responds to MSK512, EXP 513, and ENA 514 inputs to output appropriate Disable, Low,High, or Mask conditions on the encoded response output 511. Asmentioned, multiple circuits 501 will exist in the encoding circuit 407.For example, if the encoded response bus 411 contains 300 individualencoded response signals 511, 300 circuits 501 will exist in theencoding circuit 407. Also, the width of the MSK bus 410, EXP bus 409,and ENA bus 408 will be 300 signals wide each, to supply the MSK 512,EXP 513, and ENA 514 inputs to the 300 circuits 501.

Adapting Die

FIG. 6A illustrates how conventional 2-state output buffers of die 601are adapted according to the present disclosure. Die 601 is similar todie 102 in that it includes input pads 602, output pads 603, inputbuffer 604, and core circuitry 605. Die 601 differs from die 102 in thatit substitutes test circuits 606 for conventional 2-state output buffers114.

FIG. 6B illustrates test circuit 606 in more detail. Test circuit 606comprises a 3-state output buffer 607 coupled between the core output610 and output pad 603, and a compare circuit 608. Compare circuit 608inputs the core output signal 610, an input 614 from the output pad 603,a scan input signal 611, scan control signals 612, a test enable signal609, and a compare strobe signal 613. Compare circuit 608 outputs a scanoutput signal 615. The test enable signal 609 is also connected to thecontrol input of the 3-state output buffer 607. Test enable 609, scancontrol 612, and compare strobe 613 are inputs to the die 601 fromtester 401 via stimulus bus 111. Scan input 611 and scan output 615 ofmultiple compare circuits 608 are daisy-chained to allow the tester 401to serially input and output to multiple compare circuits 608 viastimulus bus 111 and response bus 112. It should be noted that in thisexample that output buffer 607 operates functionally as a 2-state outputbuffer. The reason buffer 607 is selected to be a 3-state type outputbuffer is for when test circuit 606 is placed into a test mode by thetest enable input 609.

During functional operation of the die, test enable 609 is low whichenables output buffer 607 and disables compare circuit 608. Infunctional mode, test circuit 606 operates as a conventional 2-stateoutput buffer from die 601. During test mode operation of the die, testenable 609 is high which disables output buffer 607 and enables comparecircuit 608. In test mode, test circuit 606 stops operating as aconventional 2-state output buffer and starts operating in the test modeas defined by the present disclosure. During test mode, tester 401inputs encoded response signals from the encoded response bus 411 tocompare circuit 608 via the output pad 603 and connection 614.

FIG. 7A illustrates the compare circuit 608 in more detail. Comparecircuit 608 comprises trinary gate 701, exclusive OR (XOR) gate 702, ANDgate 703, and pass/fail scan memory 704. Trinary gate 701 inputs anencoded response signal 511 from a circuit 501 via connection 614, andoutputs an expected (EXP) data signal 705 and a mask (MSK) data signal706. XOR gate 702 inputs the core output signal 610 and the EXP dataoutput signal 705, and outputs a compare signal 707. An AND gate 708inputs the compare signal 707 and the MSK data signal 706, and outputs acompare out signal 708. Pass/fail scan memory 704 inputs the compare outsignal 708, compare strobe signal 613, scan input signal 611, scancontrol signals 612, and the test enable signal 609, and outputs thescan output signal 615. The test enable signal 609 is also input totrinary gate 701, XOR gate 702, and AND gate 703. When test enable islow (i.e. functional mode of die) it disables the operation of gates701-707 such that they are not active to consume power or produce signalnoise during functional operation of the die. Also while test enable 609is low, the pass/fail latch (described below) of pass/fail scan memory704 is initialized to the pass indication state.

FIG. 7C illustrates in more detail the pass/fail scan memory 704.Pass/fail scan memory 704 comprises pass/fail latch comprising a D-FF709 (or other type of single bit memory) and OR gate 713, and a scancell comprising multiplexer 710 and D-FF 711. The pass/fail latch (i.e.Or gate 713 and FF 709) receives the compare output 708, compare strobe613, and test enable 609. Test enable 609 is input to the FF 709 resetinput to initialize FF 709 to a pass indication condition. Comparestrobe 613 is input to the FF 709 clock input. Compare out 708 and the Qoutput 712 of FF 709 are input to OR gate 713, which inputs to the Dinput of FF 709. The scan cell (i.e. multiplexer 710 and FF 711)receives the Q output 712 from FF 709, the scan input signal 611, andscan control inputs 612, and outputs the scan output signal 615.

Optionally, the scan cell may receive a boundary scan input 714 so thatthe scan cell may be used as the capture and shift stage of an IEEE1149.1 boundary scan cell in addition to its use as a pass/failindication scan cell by the present disclosure. The boundary scan input714 would be connected to core output signal 610 to allow the scan cellto capture the data output from the core then shift the captured datafrom the IC, as described in the IEEE 1149.1 standard. The scan cell isoperable in response to the scan control inputs 612 to capture thestored Q output signal 712 into FF 711 via multiplexer 710, then shiftdata from scan input 611 to scan output 615 via multiplexer 710.

The scan control inputs 612 may come from a tester as previouslymentioned, or they may be selectively connected to a test port on thedie, such as an IEEE 1149.1 test access port. When operating the scancells 704 as IEEE 1149.1 capture shift and stage elements, the scancontrol 612 to the scan cells will be coupled to the 1149.1 test accessport to allow IEEE 1149.1 control of the scan cells during boundary scantesting.

FIG. 7B depicts the operation of compare circuit 608 via a truth table.When the test enable 609 is low, compare circuit 608 is disabled exceptfor the scan cell (710, 711) that remains operable to capture and shiftdata. The reason the scan cell remains enabled is because the scan cellmay be shared between being used as a pass/fail indication scan cell bythe present disclosure and also as an IEEE 1149.1 boundary scan cellassociated with the output pad 603 of die 601, as mentioned above. Thesharing of the scan cell as both a pass/fail indication scan cell and asan IEEE 1149.1 boundary scan cell advantageously reduces test circuitarea in the die. When test enable 609 is high, the compare circuit 608is enabled to perform testing according to the present disclosure.

While test enable 609 is high, a Gnd (Low) encoded response input 614from tester 401 causes trinary gate 701 to output a high on MSK 706 anda low on EXP 705. This test condition compares for an expected low logiclevel on core output 610. If the core output 610 is low, the compareoutput 708 from gate 703 will input a low (pass condition) to pass/faillatch (713, 709). In response to the compare strobe 613 that accompanieseach encoded response input 614 from the tester 401, the low input oncompare output 708 will be clocked into FF 709 of the pass/fail latch tostore the passing compare test result.

If the core output 610 is high, the compare output 708 will input a high(fail condition) to the pass/fail latch. Again, in response to theaccompanying compare strobe 613, the high input on compare output 708will be clocked into FF 709 to store the failing compare test result. Ifa high (a fail condition) is clocked into FF 709, FF 709 will latch upwith a high (fail condition) on its Q output, via the connection 712 toOR gate 713, and remain latched high through out the remainder of test.This latch up is required to prevent the high (fail condition) frombeing overwritten during subsequent compare strobe inputs 613 to FF 709.This compare low operation of the present disclosure realizes thecompare low operation described in regard to tester 105 of FIGS. 1C and2.

While test enable 609 is high, a Vdd (High) encoded response input 614from tester 401 causes trinary gate 701 to output a high on MSK 706 anda high on EXP 705. This test condition compares for an expected highlogic level on core output 610. If the core output 610 is high, thecompare output 708 from gate 703 will input a low (pass condition) topass/fail latch (713, 709). In response to the accompanying comparestrobe 613 the low input on compare output 708 will be clocked into FF709 of the pass/fail latch to store the passing compare test result. Ifthe core output 610 is low, the compare output 708 will input a high(fail condition) to the pass/fail latch. Again, in response to theaccompanying compare strobe 613, the high input on compare output 708will be clocked into FF 709 to store the failing compare test result.

As mentioned above, if a high (a fail condition) is clocked into FF 709,the pass/fail latch will latch up through out the remainder of the testto prevent the high failing condition from being overwritten duringsubsequent compare strobe inputs 613 to FF 709. This compare highoperation of the present disclosure realizes the compare high operationdescribed in regard to tester 105 of FIGS. 1C and 2.

While test enable 609 is high, a ½Vdd (Mask) encoded response input 614from tester 401 causes trinary gate 701 to output a low on MSK 706. Thelow on MSK 706 forces the compare out 708 output of AND gate 703 low,which forces a pass condition to be clocked into the pass/fail latch,independent of the logic level output 707 from XOR gate 702. The testerinputs a ½Vdd (Mask) encoded response input to trinary gate 701 wheneverit is not desired to perform a compare operation against the logic levelon core output 610. This mask operation of the present disclosurerealizes the mask operation described in regard to tester 105 of FIGS.1C and 2.

FIG. 8A illustrates an example trinary gate 701 circuit. Trinary gate701 comprises p-channel transistor 801, current source 802, currentsource 803, n-channel transistor 804, OR gate 805, inverter 806, andtransmission gate switches 807 and 808. Transistor 801 and currentsource 802 form a first path between Vdd and Gnd. Transistor 804 andcurrent source 803 form a second path between Vdd and Gnd. A first nodebetween transistor 801 and current source 802 is connected to aninverted input of OR gate 805. A second node between transistor 804 andcurrent source 803 is connected to the other input of OR gate 805 and toinverter 806. The output of OR gate 805 is the Mask (MSK) Data signal706. The output of inverter 806 is the Expected (EXP) Data signal 705.The test enable signal 609 is connected as a control input to switches807 and 808. When test enable 609 is low, switch 807 connects the gateinput of transistor 801 to Vdd and switch 808 connects the gate input oftransistor 804 to Gnd, turning both transistor off and setting the firstand second nodes low and high respectively. When test enable 609 ishigh, switches 807 and 808 connect the gate inputs of transistors 801and 804 to the encoded response signal 614, enabling the transistors torespond to the encoded response signal.

FIG. 8B depicts the operation of trinary gate 701 via a truth table.When the test enable 609 is low, transistors 801 and 804 are disabledfrom responding to the encoded response signal 614 and the MSK 706 andEXP 705 outputs are forced high and low respectively. While test enable609 is low, the trinary gate 701 is disabled to reduce power consumptionand noise during functional mode of the die, as previously mentioned.While test enable 609 is high, and when a Gnd (Low) signal is input onthe encoded response input 614, the first and second nodes are high,producing a high on MSK signal 706 and a low on EXP signal 705. Whiletest enable 609 is high, and when a Vdd (High) signal is input on theencoded response input 614, the first and second nodes are low,producing a high on MSK signal 706 and a high on EXP signal 705. Whiletest enable 609 is high, and when a ½Vdd (Mask) signal is input on theencoded response input 614, the first node is high and the second nodeis low, producing a low on MSK signal 706 and a high on EXP signal 705.During a ½Vdd (Mask) input, the logic level output on the EXP 705 signalis indicated in the truth table as a don't care (X) since the compareoperation is masked by the low on MSK signal 706.

While not shown, the test enable signal 609 input to XOR gate 702 andAND gate 703 can be used to disable their input threshold transistorsand set their outputs to static DC low states similar to the way it isshown doing so in the trinary gate 701 of FIG. 8A. Again, this is doneto reduce power and noise of comparators 608 during functional operationof die 601.

FIG. 9A illustrates how conventional 3-state output buffers of die 601are adapted according to the present disclosure. Die 601 of FIG. 9A isthe same as die 601 of FIG. 6A with the exception that FIG. 9Aillustrates how test circuits 906 are substituted for conventional3-state output buffers between core 605 and 3-state output pads 903.Similar to die 601 of FIG. 6A, die 601 of FIG. 9A includes input pads602, input buffers 604, core circuitry 605, and 3-state output pads 903as opposed to 2-state output pads 603 in FIG. 6A. Die 601 of FIG. 9Adiffers from die 601 of FIG. 6A in that it illustrates the substitutionof test circuits 906 for conventional 3-state output buffers at outputpads 903, instead of the substitution of test circuits 606 forconventional 2-state output buffers at pads 603.

FIG. 9B illustrates test circuit 906 in more detail. Test circuit 906comprises a 3-state output buffer 907 coupled between the core output910 and output pad 903, an AND gate 901, and a compare circuit 908. ANDgate 901 receives an output control signal 911 from core 605 on oneinput and an inverted test enable signal 609 on the other input. The ANDgate 901 outputs a 3-state control signal 902 to the 3-state buffer 907.Compare circuit 908 inputs the core output signal 910, core outputcontrol signal 911, an input 914 from the output pad 903, a scan inputsignal 611, scan control signals 612, a test enable signal 609, and acompare strobe signal 613. Compare circuit 908 outputs a scan outputsignal 615. Scan input 611 and scan output 615 of multiple comparecircuits 908 and 608 are daisy-chained to allow the tester 401 toserially input and output to multiple compare circuits 908 and 608 viastimulus bus 111 and response bus 112. It should be noted that in thisexample that output buffer 907 operates functionally as a 3-state outputbuffer, as opposed to output buffer 607 of FIG. 6B which operatesfunctionally as a 2-state output buffer. As with buffer 607, the outputof buffer 907 is disabled when test circuit 906 is placed into a testmode by the test enable input 609, via AND gate 901.

During functional operation of the die, test enable 609 is low whichenables output control signal 911 from core 605 to pass through gate 901to functionally enable and disable output buffer 907. In this example,and during functional operation, a low input on output control 911 willdisable the output of output buffer 907, and a high input on outputcontrol 911 will enable the output of output buffer 907. When outputcontrol in low, Also a low on test enable 609 disables compare circuit908. In functional mode, test circuit 906 operates as a conventional3-state output buffer from die 601. During test mode operation of thedie, test enable 609 is high which disables output buffer 907, via gate901, and enables compare circuit 908. In test mode, test circuit 906stops operating as a conventional 3-state output buffer and startsoperating in the test mode as defined by the present disclosure. Duringtest mode, tester 401 inputs encoded response signals from the encodedresponse bus 411 to compare circuit 908 via the output pad 903 andconnection 914.

FIG. 10A illustrates the compare circuit 908 in more detail. Comparecircuit 908 comprises trinary gate 701, XOR gate 702, AND gate 1003, andpass/fail scan memory 704. Trinary gate 701 inputs an encoded responsesignal 511 from a circuit 501 via connection 914, and outputs anexpected (EXP) data signal 705 and a mask (MSK) data signal 706. XORgate 702 inputs the core output signal 910 and the EXP data outputsignal 705, and outputs a compare signal 707. AND gate 708 inputs thecompare signal 707, output control signal 911, and the MSK data signal706, and outputs a compare out signal 1008. Pass/fail scan memory 704inputs the compare out signal 1008, compare strobe signal 613, scaninput signal 611, scan control signals 612, and the test enable signal609, and outputs the scan output signal 615. The test enable signal 609is also input to trinary gate 701, XOR gate 702, and AND gate 1003 toreduce power consumption and noise during functional die operation, asdescribed previously in regard to comparator 608. The pass/fail scanmemory operates as previously described in regard to FIG. 7C.

FIG. 10B depicts the operation of compare circuit 908 via a truth table.When the test enable 609 is low, compare circuit 908 is disabled exceptfor the scan cell (710, 711) of pass/fail scan memory 704 to enablesharing of the scan cell as both a pass/fail indication scan cell and asan IEEE 1149.1 boundary scan cell as mentioned in regard to FIG. 7C.When test enable 609 is high, the compare circuit 908 is enabled toperform testing according to the present disclosure.

While test enable 609 and output control 911 is high, a Gnd (Low)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a high on MSK 706 and a low on EXP 705. This test conditioncompares for an expected low logic level on core output 910. If the coreoutput 910 is low, the compare output 1008 from gate 1003 will input alow (pass condition) to pass/fail latch (713, 709). In response to theaccompanying compare strobe 613, the low input (pass condition) isstored into the pass/fail latch, as previously described in regard toFIG. 7C. If the core output 910 is high, the compare output 1008 willinput a high (fail condition) to the pass/fail latch. In response to theaccompanying compare strobe 613 the high (fail condition) is stored andlatched in pass/fail latch as previously described in regard to FIG. 7C.

While test enable 609 and output control 911 is high, a Vdd (High)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a high on MSK 706 and a high on EXP 705. This test conditioncompares for an expected high logic level on core output 910. If thecore output 910 is high, the compare output 1008 from gate 1003 willinput a low (pass condition) to pass/fail latch (713, 709). In responseto the accompanying compare strobe 613, the low input (pass condition)is stored into the pass/fail latch, as previously described in regard toFIG. 7C. If the core output 910 is low, the compare output 1008 willinput a high (fail condition) to the pass/fail latch. In response to theaccompanying compare strobe 613 the high (fail condition) is stored andlatched in the pass/fail latch as previously described in regard to FIG.7C.

While test enable 609 and output control 911 is high, a ½Vdd (Mask)encoded response input 914 from tester 401 causes trinary gate 701 tooutput a low on MSK 706. The low on MSK 706 forces the compare out 1008output of AND gate 1003 low, which forces a low (pass condition) to bestored into the pass/fail latch in response to the accompanying comparestrobe 613, independent of the logic level output 707 from XOR gate 702.The tester inputs a ½Vdd (Mask) encoded response input to trinary gate701 whenever it is not desired to perform a compare operation againstthe logic level on core output 910, as previously described in regard toFIGS. 7A and 7B.

While test enable 609 is high and output control 911 is low, a low (passcondition) is forced on the compare output 1008 of AND gate 1003. Thisforces a low (pass condition) to be stored into the pass/fail latch inresponse to the accompanying compare strobe 613, independent of thelogic level output 707 from XOR gate 702. This forced pass condition isdifferent from the forced pass condition controlled by tester 401 usingthe ½Vdd input, since the core's output control signal 911 regulates themasking of the compare operation.

This new mode of compare masking enables testing the core's outputcontrol signal 911. For example, if, during a time in the test when theoutput control signal 911 should be low, an intentionally failingencoded response signal 914 can be input to the trinary gate 701. If thecontrol output signal 911 is functioning properly, it will mask theintentional failure input and force the compare output 1008 of gate 1003low (pass condition). However, if the output control signal 911 fails tofunction properly, it will not mask the intentional failure input andthe compare output signal 1008 will be set high (fail condition). Thereis a possibility that a faulty core output signal 910 may compare equalto the intentional failure input signal 914, which will mask the testfor a faulty output control signal 911.

For example, a faulty output control signal 911 may remain high (firstfault) to allow a faulty core output signal 910 to pass the compare test(second fault) and input a low (pass condition) to the pass/fail latch.To test for this possibility, two tests are run. A first test using theintentional failure input, and a second test using the actual expecteddata input. If both tests pass, then both the output control signal 911and core output signal 910 are functioning properly.

FIG. 11A illustrates how conventional input/output (I/O) buffers of die601 are adapted according to the present disclosure. Similar to die 601of FIG. 9A, die 601 of FIG. 11A includes input pads 602, input buffers604, core circuitry 605, and I/O pads 1103 as opposed to 2-state and3-state output pads 603 and 903 in FIGS. 6A and 9A. Die 601 of FIG. 11Adiffers from die 601 of FIGS. 6A and 9A in that it substitutes testcircuits 1106 for conventional I/O buffers at output pads 1103, insteadof the substitution of test circuits 606 and 906 for conventional2-state and 3-state output buffers at pads 603 and 903.

FIG. 11B illustrates test circuit 1106 in more detail. Test circuit 1106comprises a 3-state output buffer 907 coupled between core output 1110and I/O pad 1103, an input buffer 1115 coupled between I/O pad 1103 andcore input 1112, an AND gate 901, and a compare circuit 908. AND gate901 receives an I/O control signal 1111 from core 605 on one input andan inverted test enable signal 609 on the other input. The AND gate 901outputs a 3-state control signal 902 to the 3-state buffer 907. Comparecircuit 908 inputs the core output signal 1110, core I/O control signal1111, an input 1114 from I/O pad 1103, a scan input signal 611, scancontrol signals 612, a test enable signal 609, and a compare strobesignal 613. Compare circuit 908 outputs a scan output signal 615. Scaninput 611 and scan output 615 of multiple compare circuits 908 and 608are daisy-chained to allow the tester 401 to serially input and outputto multiple compare circuits 908 and 608 via stimulus bus 111 andresponse bus 112.

FIG. 12B shows the compare circuit 908 of FIG. 11B in more detail. Thestructure and operation of compare circuit 908 of FIG. 12A is the sameas compare circuit 908 of FIG. 10A. The only structural differencebetween the two compare circuits 908 is that the I/O control signal 1111of FIG. 12A has been substituted for the output control signal 911 ofFIG. 10A.

In FIG. 12B, compare circuit 908 of FIG. 12A performs all the truthtable functions of compare circuit 908 of FIG. 10A. In addition to thesefunctions, compare circuit 908 of FIG. 12A supports the input stimulusfunction described below.

During conventional testing, tester 105 of FIG. 1C inputs stimulus viastimulus bus 111 and outputs response via response bus 110 toconventional IC I/O pads. During testing according to the presentdisclosure, tester 401 of FIG. 4 inputs stimulus using either stimulusbus 414 or encoded response bus 411, and outputs encoded response viaencoded response bus 411 to IC I/O pads 1103. In either test case, theI/O control signal 1111 will select the input or output function bycontrolling the output condition of 3-state buffer 907. For example,when the I/O control signal 1111 of test circuit 1106 in FIG. 11B is setlow, the output of the 3-state buffer 907 is disabled to allow thetester 401 to input stimulus to core 605 from I/O pad 11103.

The stimulus input from the tester 401 is input using conventional logiclow (Gnd) and high (Vdd) voltage levels, which as mentioned can comefrom either the stimulus bus 414 or encoded response bus 411. As seen inFIG. 12A, the low on I/O control signal 1111 that selects the stimulusinput mode also forces the output 1008 of AND gate 1003 low to inputpass conditions to pass/fail flag in pass/fail scan memory 704. This isdone to prevent a high (fail condition) from being unintentionallystored and latched in the pass/fail flag, in response to accompanyingcompare strobes 613, during times when the tester 401 is inputtingstimulus.

As mentioned previously in regard to FIG. 3, production testing ofmultiple die preferably runs to completion without regard to one or moredie incurring failures during the test. However, during diagnostictesting of multiple die it is advantageous to be able to detect a firstfailure to allow determining the exact test pattern that caused thefailure. To provide for diagnostic testing using the present disclosure,the pass/fail scan memory 704 is modified as follows.

In FIG. 13A, the pass/fail scan memory 704 is shown to include anadditional transistor 1301. The transistor has one terminal connected toGnd and the other terminal connected to a fail output signal 1302, whichis externally output from the pass/fail scan memory 704. The gate inputof transistor 1301 is connected to the Q output signal 712 of FF 709.While the Q output 712 is low (pass condition), the transistor is offand the fail output signal 1302 is isolated from Gnd. When the Q outputis high (fail condition), the transistor is on and a conduction path isenabled between fail output signal 1302 and Gnd. As can be seen,transistor 1301 operates as an open drain, isolating the fail outputsignal 1302 from Gnd while Q is low (pass condition), and connecting thefail output signal 1302 to Gnd when Q is high (fail condition).

FIG. 13B illustrates a die 1303 coupled to a tester 401. Die 1303includes mixtures of the previously described test circuits 608 and 908.The test circuits 608 and 908 each contain the pass/fail fail output1302 equipped scan memory 704 of FIG. 13A. The fail outputs 1302 of eachtest circuit 608 and 908 are externally available to be connected to abussed fail output signal 1304 within the die. The bussed fail outputsignal 1304 is also connected to a current source 1305, which serves asa pull element for the bussed fail output signal 1304. The bussed failoutput signal 1304 is externally output from the die as a fail output totester 401. While the pull up element 1304 is shown existing inside thedie, it could exist external of the die as well, i.e. the tester 401could provide the pull up element 1305.

Diagnostic testing of multiple die 1303 using the present disclosure issimilar to the previously described production test using the presentdisclosure. However, unlike production testing, diagnostic testing willbe halted upon the first compare failure to enable identification of thedie test pattern that failed, so that the nature of the failure may beanalyzed. During diagnostic testing, the test circuits 608, 908 of themultiple die perform the compare operations between the core outputs610, 910, 1110 and encoded response inputs 614, 914, 1114.

As can be seen from FIG. 13A, when a first high (fail condition) isstored and latched in FF 709, the gate of transistor 1301 is driven highby the Q output of FF 709. With the gate input high, the transistor 1301is on and forms a conduction path between fail output 1302 and Gnd. Ascan be seen in FIG. 13B, when one or more transistors 1301 turn on inresponse to a fail condition, the bussed fail output connection 1304 ispulled low (Gnd) The tester responds to this low level transition on thefail output to halt the diagnostic test and to scan out the pass/failflags of the daisy-chained test circuits 608/908. By inspecting thescanned out pass/fail flag bits, the tester can determine which one ormore core output signal(s) failed. Thus the present disclosure supportsdiagnostic testing of multiple die if the pass/fail scan memory 704 ofFIG. 13A is used in place of the previously described pass/fail scanmemory 704 of FIG. 7C.

FIG. 14 illustrates a test system according to the present disclosure.The test system comprises a tester 401, a multiple die probe mechanism1401, and dies 1-N to be tested. The probe mechanism 1401 is similar tothe probe mechanism 301 of FIG. 3 in that it has a stimulus channel 302for probing all common input pads 303 of die 1-N. Probe mechanism 1401differs from probe mechanism 301 in that it has an encoded responsechannel for probing all common output pads 1402 of die 1-N. During test,all die 1-N receive a common stimulus input on each common input padinput 303, and all die 1-N receive a common encoded response input oneach common output pad 1402.

From inspection of the probe mechanism 1401, the test system of thepresent disclosure does not suffer from the previously mentioned testerresponse channel limitation mentioned in regard to the conventional testsystem of FIG. 3. For example, if the tester 401 has 300 stimuluschannels and 300 response channels, and die 1-N have 300 or less inputpads and 300 or less output pads, any number of die 1-N may besimultaneously tested using the test system of the present disclosure.Thus, use of the test system of FIG. 14 reduces the test time of the dieon wafer, and therefore reduces the cost to manufacture the die.

FIG. 15 illustrates an alternate view of the test system of FIG. 14.Tester 401 is illustrated as the outer layer, probe mechanism 1401 isillustrated as being inside the tester 401 layer, and wafer 1501 withdie 1-N is illustrated as being inside the probe mechanism layer 1401.Each die 1-N are identical and each have inputs 1-M connected to inputpads 1502-1504 and 2-state outputs 1-N connected to output pads1505-1507.

The stimulus bus 414 from the tester passes through the probe mechanismto the die input pads 1502-1504. The encoded response bus 411 andresponse bus 110 from the tester pass through the probe mechanism to thedie output pads 1505-1507. Common input pads 1502 of die 1-N areconnected together and to one stimulus channel from stimulus bus 414 viathe probe mechanism, common input pads 1503 are connected together andto another stimulus channel from stimulus bus 414 via the probemechanism, and inputs pads 1504 are connected together and to a furtherstimulus channel from stimulus bus 414 via the probe mechanism. Commonoutput pads 1505 of die 1-N are connected together and to one encodedresponse channel from encoded response bus 411 via the probe mechanism,common output pads 1506 are connected together and to another encodedresponse channel from encoded response bus 411 via the probed mechanism,and common output pads 1507 are connected together and to a furtherencoded response channel from encoded response bus 411 via the probemechanism.

The pass/fail scan input 611 from the tester passes through the probemechanism 1401 to the scan input of die 1, through the daisy-chainedscan path of die 1-N to be output on the pass/fail scan output 615 tothe tester via probe mechanism 1401. The scan input 611 uses one of thestimulus input channels of stimulus bus 414 and the scan output uses oneof the response output channels of response output bus 110.

While the scan control signals 612, test enable signal 609, and comparestrobe signal 613 are not explicitly shown in FIG. 15, they are alsoconnected to die 1-N inputs 1-M via stimulus channels from stimulusinput bus 414. While test circuits 606 are shown existing on die 1-N2-state output pads 1505-1507, it should be clear that test circuits 906would exist on die 1-N 3-state output pads 1505-1507, and test circuits1106 would exist on die 1-N I/O pads 1505-1507.

If test circuits 1106 were used on die I/O pads 1505-1507, then theencoded response bus 411 would be used to input stimulus data to the I/Opads 1505-1507, via probe mechanism 1401, as described in regard toFIGS. 12A and 12B. Thus is this example, the encoded response bus 411serves the dual role of, (1) inputting encoded response signals to I/Opads during compare/mask operations, and (2) inputting stimulus data toI/O pads during stimulus input operations.

FIG. 16 illustrates in detail the functional testing of die 1-N(1601-1603) of FIG. 15. Tester 401 inputs stimulus from stimulus bus 414to common die inputs 1502-1504 via the connections 1609-1611, to allowall die 1-N to receive the same stimulus at their common inputs duringtest. Connections 1609-1611 are provided by the probe mechanism 1401 ofFIGS. 14 and 15. Also, tester 401 inputs stimulus from stimulus bus 414to the scan input 611 of die 1 via the probe mechanism.

Tester 401 inputs encoded response inputs from encoded response bus 411to common die outputs and I/Os 1505-1507 via the connections 1606-1608,to allow all die 1-N to receive the same encoded response inputs attheir common outputs and I/Os during test. Connections 1606-1608 areprovided by the probe mechanism 1401 of FIGS. 14 and 15. Tester 401inputs a combined fail output signal from die 1-N to response bus 110via the fail output connection 1605 provided by the probe mechanism.Also, tester 401 inputs the scan output signal 615 from die N to theresponse bus 110. Connection 1604 illustrates the daisy-chaining of thepass/fail scan output from die 1 to the pass/fail scan input of die 2,and so on to die N. Connection 1604 is provided by the probe mechanism.As seen in FIG. 16, encoded response input 1505 is coupled to 1-N2-state test circuits 606, encoded response input 1506 is coupled to 1-N3-state test circuits 906, and encoded response input 1507 is coupled to1-N I/O test circuits 1106.

During test, tester 401 places the die 1-N in the test mode of thepresent disclosure and inputs stimulus patterns to die 1-N inputs viaconnections 1502-1504 and inputs encoded response patterns to die 1-Ntest circuits 606, 906, and 1106 via connections 1505-1507. In responseto the functional patterns to the inputs and I/Os, die 1-N operates tooutput data to test circuits 606, output data and control to testcircuits 906, and input and output data and control to test circuits1106. During the test, tester 401 inputs the compare strobe to testcircuits 606, 906, and 1106 as previously described to store the compareresults between the functional output data and the encoded responseinput data from the tester.

If the test is a production test, the fail output from connection 1605is ignored during the test for the reasons previously mentioned inregard to FIG. 3. If the test is a diagnostic test, the fail output fromconnection 1605 is monitored by the tester 401 for the reasonspreviously mentioned in regard to FIGS. 13A and 13B. At the end of afunctional production test or at the stopping of a functional diagnostictest, tester 401 scans out the pass/fail flags in the pass/fail scanmemories of die 1-N via the scan input 611 and scan output 615connections. From the pass/fail scan operation; the tester can determineif a failure occurred in die 1-N and if so identify the location of thefailure.

FIG. 17 illustrates in detail the parallel scan testing of die 1-N(1701-1703) of FIG. 15. The difference between die 1-N of FIG. 16 anddie 1-N of FIG. 17 is that die 1-N of FIG. 17 have been designed to betested using a parallel scan design for test approach, whereas die 1-Nwere not and had to be tested functionally. When die 1-N are placed inthe parallel scan test configuration, the data inputs of scan paths 1-Nare connected to die inputs 1502-1504 and the data outputs of scan paths1-N are connected to the inputs 910 of test circuits 606. Tester 401inputs scan stimulus from bus 414 to die 1-N scan paths 1-N via thecommon die input connections 1502-1504 and 1609-1611, to allow all die1-N to receive the same scan stimulus during test. Also, tester 401inputs stimulus from bus 414 to the scan input 611 of die 1 via theprobe mechanism.

Tester 401 inputs encoded scan response from bus 411 to common dieoutput connections 1505-1507 and 1606-1608, to allow all die 1-N tocompare against the same response during test. Tester 401 inputs acombined fail output signal from die 1-N to response bus 110 via thefail output connection 1605. Also, tester 401 inputs the scan outputsignal 615 from die N to the response bus 110. Connection 1604illustrates the daisy-chaining of the pass/fail scan output from die 1to the pass/fail scan input of die 2, and so on to die N. As seen inFIG. 17, encoded scan response inputs 1505-1507 are coupled to 1-N2-state test circuits 606.

During test, tester 401 places the die 1-N in the test mode of thepresent disclosure and inputs stimulus patterns to scan paths 1-N of die1-N via inputs 1502-1504 and inputs encoded response patterns to testcircuits 606 of die 1-N via outputs 1505-1507. The scan paths operate,in response to conventional scan path control input from tester 401, toshift in the stimulus patterns from inputs 1502-1504, capture responsepatterns, and shift out the captured response patterns to test circuits606. During the test, tester 401 inputs the compare strobe to testcircuits 606 as previously described to store the compare resultsbetween the captured response data from scan paths 1-N and the encodedresponse input data from tester 401. If the test is a production test,the fail output from connection 1605 is ignored during the test for thereasons previously mentioned in regard to FIG. 3.

If the test is a diagnostic test, the fail output from connection 1605is monitored by the tester 401 for the reasons previously mentioned inregard to FIGS. 13A and 13B. At the end of a parallel scan productiontest or at the stopping of a parallel scan diagnostic test, tester 401scans out the pass/fail flags in the pass/fail scan memories of die 1-Nvia the scan input 611 and scan output 615 connections. From thepass/fail scan operation; the tester can determine if a failure occurredin die 1-N and if so identify the location of the failure.

It is becoming increasingly popular to design systems on ICs usingpre-existing intellectual property core sub-circuits. Core sub-circuitsprovide embeddable functions such as DSP, CPU, and RAM.

FIG. 18A illustrates an IC comprising embedded cores 1-3. The cores areconnected together via functional connections 1814 and 1815 to form asystem on the IC. The following describes how such systems on ICs can betested using the present disclosure.

To test the embedded cores 1-3 of IC 1802, test connections 1810 andconnection circuits 1808 and 1809 are added to allow input pads 1803 tobe selectively connected to the inputs of cores 1-3. Also testconnections 1811, 1812 and 1818 are added to allow the outputs of cores1-3 to be connected to test circuits 1813, which are coupled to outputpads 1802.

In FIG. 18B, test circuit 1813 is similar to test circuit 606 with theexception that it contains a multiplexer 1816 for receiving core 1-3outputs 1811, 1812, and 1818 and a core select input 1817 for selectingwhich of the core 1-3 outputs 1811, 1812, or 1818 will be selected forinput to buffer 607 and compare circuit 608.

During the testing of core 1, the IC of FIG. 18A is configured such thatthe inputs to core 1 are coupled to input pads 1803 and the outputs fromcore 1 are coupled to test circuits 1813 via connections 1811. Also testcircuit 1813 is configured by the core select signals 1816 to connectthe core 1 outputs to compare circuits 608. After the IC has beenconfigured, core 1 is rendered testable using the present disclosure byinputting stimulus to core 1 via pads 1803 and inputting encodedresponse to test circuit 1813 via pads 1802 to compare against theoutputs from core 1. The testing of core 1 is as previously described inFIGS. 6A and 6B.

During the testing of core 2, the IC of FIG. 18A is configured such thatthe inputs to core 2 are coupled to input pads 1803, via connection 1810and connection circuit 1808, and the outputs from core 2 are coupled totest circuits 1813 via connections 1812. Also test circuit 1813 isconfigured by the core select signals 1816 to connect the core 2 outputsto compare circuits 608. After the IC has been configured, core 2 isrendered testable using the present disclosure by inputting stimulus tocore 2 via pads 1803 and inputting encoded response to test circuit 1813via pads 1802 to compare against the outputs from core 2. The testing ofcore 2 is as previously described in FIGS. 6A and 6B.

During the testing of core 3, the IC of FIG. 18A is configured such thatthe inputs to core 3 are coupled to input pads 1803, via connection 1810and connection circuit 1809, and the outputs from core 3 are coupled totest circuits 1813 via connections 1818. Also test circuit 1813 isconfigured by the core select signals 1816 to connect the core 3 outputsto compare circuits 608. After the IC has been configured, core 3 isrendered testable using the present disclosure by inputting stimulus tocore 3 via pads 1803 and inputting encoded response to test circuit 1813via pads 1802 to compare against the outputs from core 3. The testing ofcore 3 is as previously described in FIGS. 6A and 6B.

The individual core 1-3 tests described above could be performedsimultaneously on multiple ICs of FIG. 18A as described in regard FIG.15, which would lower the cost to manufacture the ICs of FIG. 18A.

FIG. 19 illustrates a wafer 1901 that has been processed to includebuilt-in connections for accessing common die input (S1) and common dieoutput (R1) pads. The wafer comprises; (1) die 1-N each with input (S1)pads and output (R1) pads, (2) stimulus input grid lines 1904 connectedto common die input pads, (3) encoded response input grid lines 1905connected to common die output pads, (4) pad fuses 1906 connected inseries between grid lines 1905 and pad connection lines 1907, 1908,1909, and 1910, (5) tester probe contacts 1903 for connecting tostimulus grid lines 1904, and (6) tester probe contacts 1902 forconnecting to encoded response grid lines 1905.

Tester 401 probes grid line contacts 1903, 1902 using a simplifiedexternal probe mechanism to input stimulus to the commonly connected dieinput pads and to input encoded response to the commonly connected dieoutput pads. Testing occurs on the die as previously described. Thedifference between the test systems of FIG. 19 and FIG. 14 is that inFIG. 19 most of the common pad connections are provided on the wafer1901, whereas in FIG. 14 most of the common pad connections are providedby the external probe mechanism 1401.

The fuses 1906 are included between grid lines 1905 and common padconnections 1907-1910 to provide for the case where a faulty die outputcannot be disabled by the test enable signal 609. For example, if thetester 401 sets the test enable signal 609 high to enable testing usingthe present disclosure, and the output pad of die 3 remains enabledoutputting a logic level, the fuse 1906 between gird line 1905 and theenabled output pad of die 3 will blow whenever the tester inputs anoppose logic level on grid line 1905. Without the fuse, the logic levelmaintained on the output pad of die 3 could prevent testing of the otherdie on wafer due to logic state contention on grid line 1905.

Alternatively, a resistive element could be substituted for each fuse1906 to provide current limiting between a faulty die output pad andtester to enable testing of the other die. After testing and prior tothe die singulation step, the pad connecting grid lines, probe contacts,and fuses/resistive elements can be polished off the wafer 1901.

If wafers were processed to include the embedded pad connection schemeshown on wafer 1901 of FIG. 19, tester 401 could probe multiple ones ofthe wafers 1901 at common probe contacts 1903 and 1902 to enablesimultaneous testing of multiple wafers 1901. Being able to testmultiple wafers simultaneously using one tester 401 would bring aboutfurther reductions in test time and cost of manufacturing die.

An example of the above described multiple wafer test approach would bewhere tester 401 makes contact to probe contacts 1903 and 1902 ofmultiple wafers 1901 via the previously described multiple wafer probemechanism 1401 of FIG. 14. The major difference from FIG. 14 is that inFIG. 14 multiple die are tested whereas in this example, multiple wafersare tested.

While the present disclosure has been described thus far as being usedto simultaneously test multiple die on wafer and, as mentioned in regardto FIG. 19, even multiple wafers, it can also be used to simultaneouslytest multiple packaged ICs as well.

FIG. 20 illustrates a test system according to the present disclosurefor simultaneously testing multiple packaged ICs 1-N. The test systemcomprises a tester 401, a multiple IC probe mechanism 2001, andidentical packaged ICs 1-N to be tested. In this example, ICs 1-N eachcomprise a die 601, a package 2002 for holding die 601, bond wires 2003for connecting the output pads 603 of die 601 to package output leads2004, and bond wires 2005 for connecting input pads 602 of die 601 topackage input leads 2006.

The process of testing ICs 1-N in FIG. 20 is the same as that describedin the testing of die 1-N in FIG. 14. The only difference between thetwo tests is that the packaged die 601 of FIG. 20 are connected to theIC probe mechanism 2001 via bond wires 2005 and 2003 and input andoutput package leads 2006 and 2004. It is assumed in FIG. 20 that eachIC 1-N has package leads available for the test enable 609, scan control612, scan input 611, scan output 615, compare strobe 613, and failoutput 1302 signals. However, if not all the signals are available onpackage leads, they may be provided by sharing functional package leadsor by generating the signals internal to the die using test interfacessuch as the IEEE standard 1149.1 test access port interface.

FIG. 21 illustrates a tester 2100 capable of outputting busses 2103 and2104 of mask (MSK) and expected data (EXP) signals, respectively. Thetester 2100 of FIG. 21 is the same as the tester of FIG. 4 with theexception that the mask & expected encoding circuitry 407 in FIG. 4 hasbeen replaced by the mask & expected driver circuitry 2102 in FIG. 21.Like the mask & expected encoding circuitry 407 of FIG. 4, the mask &expected driver circuitry 2102 of FIG. 21 receives MSK 410, EXP 409, andEnable (ENA) 408 inputs from memory 406. Unlike, the mask & expectedencoding circuitry 407 of FIG. 4, the mask & expected driver circuitry2102 outputs separate mask 2103 and expected data 2104 signals ratherthan encoding them into a single signal output 411, as did the mask &expected encoding circuitry 407. Thus the mask & expected drivercircuitry 2102 outputs a first bus of mask signals 2103 and a second busof expected data signals 2104.

FIG. 22A illustrates one example implementation 2201 of a mask driver2202 and an expected driver 2203 that could exist within mask & expecteddriver circuitry 2104. The mask driver 2202 and expected data driver2203 receive mask 410 and expected data 409 inputs from memory 406,respectively, and output mask 2103 and expected data 2104, respectively,to die/ICs to be tested. Also, the drivers 2202 and 2203 receive enable(ENA) 408 control from memory 406 for selectively enabling and disablingtheir outputs.

In FIG. 22B, the truth table depicts the operation of the mask andexpected data driver implementation 2201 in response to ENA 408, EXP409, and MSK 410 signal inputs. During the Compare Low operation, MSK ishigh and EXP is low. During the Compare High operation, MSK is high andEXP is high. During the Mask Compare operation, MSK is low and EXP is adon't care. As will be described later in regard to FIGS. 26A and 26B,the operations indicated by the states of the MSK 2103 and EXP 2104outputs are performed by test circuits located on the die/IC.

FIG. 23 illustrates an arrangement 2300 of an output buffer 2307connected between a core output 2310 and output pad 2312, and a testcircuit 2301 connected between a core output 610 and output pad 2313. Inthis example, core output 610 is assumed to be connected, during testmode, to the output of a scan path in the die/IC. Also in this example,core output 2310 is an output that is not needed during scan test, soits pad 2312 can be used or shared for inputting mask data during scantest.

The test circuit 2301 is similar to the test circuit 606 of FIG. 6B inthat it includes an output buffer 607, compare circuit 2314, andinterface signals 609, 610, 611, 612, 613, and 615. The differencebetween test circuit 2301 and test circuit 606 is that the comparecircuit 2314 of test circuit 2301 has separate inputs for receiving amask signal 2303 and an expected data signal 2302, instead of a singleinput for receiving an encoded mask and expected data signal 614, as didthe compare circuit 608 of test circuit 606.

When the die/IC containing the arrangement 2300 is placed in test mode,by the test enable 609 signal, the outputs of functional buffers 2307and 607 are disabled and the compare circuit 2314 is enabled. A gatingcircuit 2308 (such as And gate 2309) is shown in the connection pathbetween the test enable signal 609 and buffer 2307 to illustrate use andcontrol of functionally required 3-state buffers 2307, similar to theuse and control of functionally required 3-state buffers described inregard to FIG. 9B.

While in test mode, the tester 2100 of FIG. 21 can input mask 2103 andexpected data 2104 signals (i.e. the response signals) to output pads2312 and 2313, respectively. The mask 2103 signal is received by input2303 of test circuit 2314 via connection 2305, and the expected data2104 signal is received by input 2302 of test circuit 2314 viaconnection 2304. Simultaneously the tester inputs stimulus signals toother pads on the die/IC being tested. Since scan testing is beingperformed, the expected data signal input on pad 2313 will be comparedagainst the scan path output response from the die/IC on core outputsignal 610, unless the compare operation is masked by the mask input onpad 2312.

Testing using the arrangement 2300 of FIG. 23 is much the same as thatpreviously described using test circuit 606. The key distinction beingthat the test circuit 2301 has two inputs 2302, 2303 for receivingseparate mask and expected data signals from two pads, as opposed to oneinput for receiving a single encoded mask and expected data signal froma one pad, as did test circuit 606.

FIG. 24 illustrates an arrangement 2400 whereby an input buffer 2402 andpad 2403, as opposed to an output buffer 2307 and pad 2312 in FIG. 23,are used to provide the mask input signal to test circuit 2301. As seenin FIG. 24, the mask data input to test circuit 2301 may selectivelycome from the connection 2408 between pad 2403 and buffer 2402, or fromthe connection 2401 between the buffer 2402 and die/IC core input.Further shown in FIG. 24 is an optional gating circuit 2405 (such as Andgate 2407), that, under control of the test enable 609 signal viaconnection 2406, blocks the output of buffer 2402 from being input tothe core of the die/IC during test. During test and with the exceptionthat an input pad 2403 is used to input mask data instead of an outputpad 2312, the operation of arrangement 2400 is the same as the operationof arrangement 2300.

Arrangement 2501 of FIG. 25A and arrangement 2502 in FIG. 25B areprovided to simply illustrate that alternate mask and expected data padconnections to test circuits 2314 may be used if desired. Arrangement2501 shows output pad 2312 and connection 2305 providing a path from thetester's expected data output 2104 to the expected data input 2302 ofcompare circuit 2314, while output pad 2313 and connection 2304 providesa path from the tester's mask data output 2103 to the mask data input2303 of compare circuit 2314. Similarly, arrangement 2502 shows inputpad 2403 and connection 2404 providing a path from the tester's expecteddata output 2104 to the expected data input 2302 of compare circuit2314, while output pad 2313 and connection 2304 provides a path from thetester's mask data output 2103 to the mask data input 2303 of comparecircuit 2314.

In general and during test mode, any available die/IC pad and connectionmay be used to provide mask and expected data input from the tester tothe mask 2303 and expected data 2302 inputs of compare circuits 2314.Indeed, the pads used in supplying mask and expected data from thetester to the mask and expected data inputs of compare circuits 2314 donot even need to be pads coupled to the functional buffers 607associated with the compare circuits 2314.

Further, it should be clear that compare circuits 2314, and othersdescribed herein such as compare circuit 608 of FIG. 6B, need not becoupled to functional output buffers as shown for example in FIGS. 6Band 24, but rather only need to be coupled to a signal on the die/IC,like core output signal 610, that provides a response output that can becompared against expected data input from a tester, or alternatively,masked from being compared by input from a tester.

FIG. 26A illustrates in more detail one-example implementation ofcompare circuit 2314. Compare circuit 2314 is similar to compare circuit608 of FIG. 7A with the exception that trinary gate 701 of FIG. 7A isnot required in compare circuit 2314. In compare circuit 2314, the EXP705 and MSK 706 signals provided by trinary gate 701 of compare circuit608 are provided by inputs 2302 and 2303 in compare circuit 2314.

The truth table in FIG. 26B depicts the operation of the compare circuit2314. Contrasting the truth tables of FIG. 22B and FIG. 26B it is seenthat the test operations previously described in regard to tester's mask& expected driver circuitry 2201 of FIG. 22A are realized by the comparecircuit 2314 when a connection is formed between the tester's maskoutput 2103 and the compare circuit's mask input 2303, and between thetester's expected data output 2104 and the compare circuit's expecteddata input 2302. With the above mentioned exception, the operation ofcompare circuit 2314 is the same as the operation of compare circuit 608of FIG. 7A.

In FIG. 26C, the pass/fail scan memory 704 is the same as in previouslydescribed FIG. 7C.

FIG. 27 illustrates the previously described test system of FIG. 15adapted for use with the modified tester 2100 of FIG. 21 and themodified test circuit arrangements of FIGS. 23, 24, 25A, and 25B.

The test system of FIG. 27 is similar to the test system of FIG. 15 withthe exception that the tester 2100 provides separate mask 2103 andexpected data 2104 signal inputs to corresponding separate pad pairs2701 and 2702 which are coupled to corresponding mask 2303 and expecteddata 2302 inputs of each test circuit 2703 (i.e. test circuits 2300,2400, 2501, or 2502) within each die 1-N. Again, as mentioned in regardto FIG. 15, each common pad of die 1-N are connected together and drivenby either a stimulus 414 input from tester 2100 or response 2104, 2103inputs from tester 2100, to allow all die 1-N to be tested in parallel.

During scan testing, unlike functional testing, only a subset of commonpads on die 1-N need to be connected together and accessed for input ofstimulus and response from tester 2100. Therefore, each die has extrapads available for providing the two tester response inputs (i.e. maskand expected data inputs) to each test circuit 2703.

FIG. 28 illustrates in detail the parallel scan testing of die 1-N(2801-2803) of FIG. 27. When die 1-N are placed in the parallel scantest configuration, the data inputs of scan paths 1-N are connected todie inputs 1502-1504 and the data outputs of scan paths 1-N areconnected to the inputs 610 of test circuits 2301. Tester 2100 inputsscan stimulus from bus 414 to die 1-N scan paths 1-N via the common dieinput connections 1502-1504 and 1609-1611, to allow all die 1-N toreceive the same scan stimulus during test. Also, tester 2100 inputsfrom bus 414 to the scan input 611 of die 12801 via the probe mechanism.

Tester 2100 inputs mask data from tester bus 2103 to common die padconnections coupled to each test circuit's mask input 2303, and inputsexpected data from tester bus 2104 to common die pad connections coupledto each test circuit's expected data input 2302, as indicated byconnections 2808-2810. Any of the test circuit arrangements of FIGS. 23,24, 25A, and 25B may be used and are indicated in FIG. 28 as testcircuit arrangements 2805-2807.

Similar to the test previously described in regard to FIG. 17, thetester 2100 inputs scan stimulus and response to all die 1-Nsimultaneously. The test circuit arrangements 2805-2807 of each dieoperate to compare or mask the scan path outputs 610 of each die asdetermined by the mask 2103 and expected data 2104 input from the tester2100. Tester 2100 inputs a combined fail output signal 1302 from die 1-Nto response bus 110 via the fail output connection 1605. Also, tester2100 inputs the scan output signal 615 from die N to the response bus110.

Connection 1604 illustrates the daisy-chaining of the pass/fail scanoutput from die 1 to the pass/fail scan input of die 2, and so on to dieN. At the end of the scan test, the tester 2100 shifts the pass/failscan path of the serially connected die 1-N, from 611 to 615, to unloadthe pass/fail bits from each test circuit's pass/fail scan memory todetermine which die passed or failed.

FIG. 29 is provided to simply illustrate that parallel testing of ICs1-N 2901-2903 is possible as described above for parallel testing of die1-N 2801-2803. In FIG. 29, the tester 2100 provides stimulus input tocommonly connected IC pins 2901-2903 and response input to commonlyconnected mask IC pins and commonly connected expected data IC pins2904-2906. The testing is the same as previously described for the die1-N with the exception that the stimulus and response inputs passthrough pins and bond wires to get to the die pads as shown in FIG. 20.As in the parallel die test of FIG. 28, the tester shifts out thepass/fail scan path within each serially connected IC 1-N after test todetermine which IC passes or fails.

While FIGS. 28 and 29 illustrate the die and ICs as being seriallyconnected together via connection 1604 and to a scan input and scanoutput of the tester, each die and IC could be individually connected tothe tester via separate scan input and scan output pads/pins.Furthermore, the tester may connect to a subset of serially connecteddie and ICs using a separate scan input and scan output for each saidsubset of serially connected die and ICs. In either arrangement thepurpose would remain to be the unloading of pass/fail information fromthe test circuits at the end of testing as previously described.

FIG. 30 is provided to illustrate how the test circuit arrangement 2300of FIG. 23 could be modified to allow for the previously describedtesting of multiple embedded intellectual property cores 1805-1807within a die/IC as shown in FIG. 18A. The modification involves theaddition of multiplexer 1816 to the test circuit 2301 such that aselected core output 1811, 1812, 1818 may be coupled to the input 610 oftest circuit 2301. As previously described, the selection action ofmultiplexer 1816 is controlled by a core select input 1817. While testcircuit arrangement 2300 is shown in FIG. 30, it should be understoodthat any of the test circuit arrangements described herein, such as2400, 2501, and 2502, could be similarly used with multiplexer 1816 toachieve the testing of embedded intellectual property cores.

The application may be practiced other than as specifically described.

1. An integrated circuit comprising: A. first and second die pads; B.core circuitry having core input leads and core output leads; and C. afirst buffer having an input lead connected to a first core output lead,a tri-state enable lead, and an output lead connected to the first diepad; D. a second buffer having an input lead and an output leadconnected between the second die pad and one of a core input lead and asecond core output lead; and E. comparator circuitry having a core inputlead connected to the first core output lead, a test enable leadconnected to the tri-state enable lead, an expected data input leadconnected to one of the first die pad and the second die pad, and a maskdata input lead connected to the other of the first die pad and thesecond die pad.
 2. The integrated circuit of claim 1 in which thecomparator circuitry includes a scan input lead, a scan output lead, ascan control input lead and a compare strobe input lead.
 3. Theintegrated circuit of claim 1 in which the expected data input isconnected to the first die pad and the mask data input is connected tothe second die pad.
 4. The integrated circuit of claim 1 in which themask data input is connected to the first die pad and the expected datainput is connected to the second die pad
 5. The integrated circuit ofclaim 1 in which the second buffer is an input buffer and the mask datainput is connected to the second die pad through the second buffer. 6.The integrated circuit of claim 1 in which the second buffer is an inputbuffer and the expected data input is connected to the second die padthrough the second buffer.
 7. An integrated circuit comprising: A. afirst output die pad B. a second output die pad; C. core circuitryhaving a first core output lead and a second core output lead; D. afirst tristate output buffer having an input connected to the first coreoutput lead, a tristate input connected to a test enable lead, and adata output connected to the first output die pad; E. a second tristateoutput buffer having an input connected to the second core output lead,a tristate input, and a data output connected to the second output diepad; F. test circuitry having a core input lead connected to the firstcore output lead, a test enable input lead connected to the test enablelead, a compare strobe input lead, an expected data input lead connectedto the first output die pad, a mask data input lead connected to thesecond output die pad, a scan input lead, a scan output lead, and a scancontrol input lead; and G. a gate having a first input connected to thetest enable lead, a second input connected to a control lead, and anoutput connected to the tristate input of the second tristate outputbuffer.
 8. An integrated circuit comprising: A. an output die pad B. aninput die pad; C. core circuitry having a core output lead and a coreinput lead; D. a tristate output buffer having an input connected to thecore output lead, a tristate input connected to a test enable lead, anda data output connected to the output die pad; E. an input buffer havingan input connected to the input die pad and a data output connected tocore input lead; F. test circuitry having a core input lead connected tothe core output lead, a test enable input lead connected to the testenable lead, a compare strobe input lead, an expected data input leadconnected to the output die pad, a mask data input lead coupled to theinput die pad, a scan input lead, a scan output lead, and a scan controlinput lead; and G. a gate having a first input connected to the testenable lead, a second input connected to the data output of the inputbuffer, and an output connected to the core input lead.
 9. An integratedcircuit comprising: A. a first output die pad B. a second output diepad; C. core circuitry having a first core output lead and a second coreoutput lead; D. a first tristate output buffer having an input connectedto the first core output lead, a tristate input connected to a testenable lead, and a data output connected to the first output die pad; E.a second tristate output buffer having an input connected to the secondcore output lead, a tristate input, and a data output connected to thesecond output die pad; F. test circuitry having a core input leadconnected to the first core output lead, a test enable input leadconnected to the test enable lead, a compare strobe input lead, a maskdata input lead connected to the first output die pad, an expected datainput lead connected to the second output die pad, a scan input lead, ascan output lead, and a scan control input lead; and G. a gate having afirst input connected to the test enable lead, a second input connectedto a control lead, and an output connected to the tristate input of thesecond tristate output buffer.
 10. An integrated circuit comprising: A.an output die pad B. an input die pad; C. core circuitry having a coreoutput lead and a core input lead; D. a tristate output buffer having aninput connected to the core output lead, a tristate input connected to atest enable lead, and a data output connected to the output die pad; E.an input buffer having an input connected to the input die pad and adata output connected to core input lead; F. test circuitry having acore input lead connected to the core output lead, a test enable inputlead connected to the test enable lead, a compare strobe input lead, amask data input lead connected to the output die pad, an expected datainput lead coupled to the input die pad, a scan input lead, a scanoutput lead, and a scan control input lead; and G. a gate having a firstinput connected to the test enable lead, a second input connected to thedata output of the input buffer, and an output connected to the coreinput lead.
 11. A wafer of semiconductor material comprising: A. pluraldies of integrated circuits formed on the wafer; B. each die including acore circuit with inputs and outputs, die pads, input buffers couplingsome of the die pads to the inputs of the core circuit, and testcircuits, each test circuit having a core circuit input connected withone core circuit output, an expected data input lead coupled to one diepad, a mask data input lead coupled to another die pad separate from theone die pad, the one and the another die pads for each test circuitbeing separate from the one and the another die pads of all other testcircuits, and comparator circuits having inputs connected to the corecircuit input, expected data input lead, the expected data input lead,and the mask data input lead, and having a comparator output; and C. ascan path formed on the wafer, the scan path being serially connectedthrough all of the test circuits and in each test circuit the scan pathbeing coupled to the comparator output.
 12. An integrated circuitcomprising: A. plural die pads; B. serial scan paths, each scan pathincluding an input and an output, the input of each serial scan pathbeing coupled to a die pad; B. test circuits, each test circuit having ascan path input connected with one scan path output, an expected datainput lead coupled to one die pad, a mask data input lead coupled toanother die pad separate from the one die pad, the one and the anotherdie pads for each test circuit being separate from the one and theanother die pads of all other test circuits, and comparator circuitshaving inputs connected to the core circuit input, expected data inputlead, the expected data input lead, and the mask data input lead, andhaving a comparator output; and C. a scan path serially connectedthrough all of the test circuits and in each test circuit the scan pathbeing coupled to the comparator output.
 13. An integrated circuitcomprising: A. first and second die pads; B. core circuitry having coreinput leads and core output leads; and C. a first buffer having an inputlead, a tri-state enable lead, and an output lead connected to the firstdie pad; D. a second buffer having an input lead and an output leadconnected between the second die pad and the core; E. comparatorcircuitry having a core input lead connected to the first core outputlead, a test enable lead connected to the tri-state enable lead, anexpected data input lead connected to one of the first die pad and thesecond die pad, and a mask data input lead connected to the other of thefirst die pad and the second die pad; and F. multiplexer circuitryhaving plural inputs, a select input, and an output connected to theinput lead of the first buffer.
 14. A process of testing an integratedcircuit having die pads, comprising: A. applying stimulus data receivedon first die pads to core circuitry to produce response data; B.receiving expected response data on second die pads separate from thefirst die pads; C. receiving mask data on third die pads separate fromthe first and second die pads; D. masking the expected response datawith the mask data to produce masked expected response data; E.producing a compare signal by comparing the response data with themasked expected response data; and F. scanning the compare signal fromthe integrated circuit in a serial scan path.